GNUzilla is the GNU version of the Mozilla suite, and GNU IceCat is the GNU version of the Firefox browser. Its main advantage is an ethical one: it is entirely free software. While the Firefox source code from the Mozilla project is free software, they distribute and recommend non-free software as plug-ins and addons. Also their trademark license restricts distribution in several ways incompatible with freedom 0.

emphasized by me

I want to understand the trademark license restriction. And how Firefox is not completely free. I would also like to ask: What are the differences between Firefox and IceCat?

1 Answer
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The main problem (that also led to the development of Debian's Iceweasel) is the trademark issue, that binds the use of the name Firefox and the logo to certain requirements, that were unacceptable to Debian and GNU. For instance in the case of Debian, Mozilla demanded to upgrade the version of Firefox instead of backport bugfixes as Debian does it. These issues are avoided by renaming it and use a different logo. As the creation of Icecat shows, that is pretty easy and functionality is unaffected.

And finally another issue the GNU-project has, is also bolded in your quotation. Firefox allows proprietary plugins and extensions. Moreover, these are distributed through Mozilla's infrastructure and are recommended in the process.